Imaging Device

ABSTRACT

For an image pickup device formed from a plurality of pixels, each of which can perform any of an exposure operation and a reading operation thereof at a timing different from that of the other, an imaging apparatus is provided, which can determine the amount of main-flashing light based on pre-flashing by a flash highly accurately. 
     Before and during a preflash operation by the flash  21,  an exposure operation is started simultaneously for all the pixels of the image pickup device  13  to form an image before and during the preflash to obtain a detected value by the detector circuit  17  before and during the preflash, respectively. The computation circuit  18  computes a differential detected value obtained by subtracting the before-preflash detected value from the during-preflash detected value. The differential detected value is a detected value containing only pre-flashed light with ambient light excluded. An amount of light for main flashing is computed on the basis of the differential detected value.

TECHNICAL FIELD

The present invention relates to an imaging apparatus for forming imagesby firing a flash.

BACKGROUND ART

An imaging apparatus is known which, for flash imaging, fires apreflash, and detects reflected light from an object to determine anamount of main-flashing light. Imaging apparatus mainly use a CCD(Charge Coupled Device) image sensor as their image pickup device.Recently, amid the growing trend toward a higher density of pixels onimage pickup devices, a CMOS image sensor is drawing attention as a newtype of image pickup device. The CMOS image sensor has advantages suchas random access to and high-speed reading of pixel signals, highsensitivity, low power consumption, and the like.

However, in a conventional imaging apparatus using a CMOS image sensor,exposure operation and reading operation are performed differently foreach pixel, and when a preflash is fired, preflash effects are givenonly to a part of the image pickup device, posing a problem ofdifficulty in determining the amount of main-flashing light accuratelyenough. In order to overcome this problem, an apparatus has beenprovided which applies a sufficiently long preflash exposure time (see,e.g., Patent Document 1 (Japanese Patent Application Publication No.2000-196951).

DISCLOSURE OF THE INVENTION

However, when the imaging apparatus applying a sufficiently longpreflash exposure time fires a preflash under ambient light, the amountof light entering a certain zone of the CMOS image sensor may sometimesexceed a dynamic range of the CMOS image sensor, producing image signalsexhibiting improper levels from that zone, and thus leaving unsolved theproblem of the difficulty in determining the amount of main-flashinglight accurately enough.

The present invention has been made in view of such circumstances, andan object thereof is to provide an imaging apparatus capable ofdetermining the amount of main-flashing light based on pre-flashing by aflash highly accurately, even using an image pickup device formed from aplurality of pixels, each of which can perform any of its exposureoperation and reading operation at a timing different from that of theother, such as an XY addressable image sensor, notably a CMOS imagesensor.

In order to achieve the above object, an imaging apparatus of thepresent invention includes a flash for emitting light onto an object, animage pickup device formed from a plurality of pixels, each of which canperform any of an exposure operation and a reading operation thereof ata timing different from that of the other, a detector circuit fordetecting a brightness of image information formed by the image pickupdevice, and a control circuit for controlling operations of the imagepickup device and of the detector circuit. In the imaging apparatus, thecontrol circuit causes the flash to fire a preflash before amain-flashing operation by the flash, causes the image pickup device toform an image at the time of the preflash, and causes the detectorcircuit to detect the brightness of image information formed at the timeof the preflash, to compute an amount of main-flashing light to be firedby the flash on the basis of the detected brightness of the imageinformation formed at the time of the preflash. The control circuit ischaracterized by starting the exposure operation simultaneously for allthe pixels of the image pickup device at the time of the preflash by theflash, whereby to form the image at the time of the preflash.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a general configuration of an imagingapparatus according to Embodiment 1.

FIG. 2 is a diagram of a general configuration of a CCD image sensor.

FIG. 3 is a diagram of a general configuration of a CMOS image sensor.

FIG. 4 is a timing chart showing an operation of the CCD image sensor.

FIG. 5 is a timing chart showing an operation of the CMOS image sensor.

FIG. 6 is a diagram showing a control flow of flash imaging.

FIG. 7 is a diagram showing an example image of an object.

FIG. 8 is a diagram showing an example computation of a differentialdetected value under no ambient light.

FIG. 9 is a diagram showing an example computation of a differentialdetected value under ambient light.

FIG. 10 is a diagram showing a flash imaging sequence for a still imageby a conventional CCD sensor.

FIG. 11 is a diagram showing a flash imaging sequence for a still imageby a conventional CMOS sensor.

FIG. 12 is a diagram showing an image formed at the time of a preflashby the conventional CCD sensor.

FIG. 13 is a diagram showing an image formed at the time of a preflashby the conventional CMOS sensor.

FIG. 14 is a diagram showing a flash imaging sequence for a still imageby the CMOS sensor according to Embodiment 1.

FIG. 15 is a diagram showing an example computation of a differentialdetected value in Embodiment 1.

BEST MODES FOR CARRYING OUT THE INVENTION

In order to achieve the object of determining the amount ofmain-flashing light based on pre-flashing by a flash highly accurately,an imaging apparatus is configured such that an exposure operation isstarted simultaneously for all pixels of its image pickup device at thetime of a preflash operation to form an image at the time of thepreflash.

Embodiment 1

An imaging apparatus according to Embodiment 1 of the present inventionwill be described below with reference to the drawings.

FIG. 1 is a diagram showing a general configuration of the imagingapparatus according to Embodiment 1.

As shown in FIG. 1, the imaging apparatus according to Embodiment 1includes a lens 11, an iris 12, an image pickup device 13, an AGC (AutoGain Controller) 14, an A/D converter 15, a camera signal processingcircuit 16, a detector circuit 17, a computation unit 18, a memory unit19, a flashing circuit 20, a flash 21, a lens driver 22, and a memoryunit 23.

The lens 11 passes light from an object therethrough for focusing ontothe image pickup device 13, during imaging. The iris 12 changes itsaperture to optimize the amount of light entering through the lens 11,for the sensitivity of the image pickup device 13. Also, the iris 12functions as a shutter. The image pickup device 13 includes a pluralityof pixels with R, G, B color filters arranged therefor, andphotoelectrically converts the light having entered each of the pixelsthrough the lens 11 into an analog signal (charge). Moreover, the imagepickup device 13 is formed from an XY addressable image sensor, such as,e.g., a CMOS image sensor, and is configured such that each of theplurality of pixels perform any of its exposure operation and readingoperation at a different timing. The CMOS image sensor is advantageousin terms of its low power dissipation and high-speed reading.

The AGC 14 amplifies video signals generated by the image pickup device13. The A/D converter 15 converts the analog video signals amplified bythe AGC 14 into digital video signals. The camera signal processingcircuit 16 performs various signal processing, so far well known, on thedigital video signals converted by the A/D converter 15, and includes,e.g., a white balance circuit, a Y/C separation circuit, a filteringcircuit, an aperture controller, a gamma correction circuit, and thelike, all not shown. The detector circuit 17 detects distributionswithin the screen of brightnesses and colors contained in the videosignals processed by the camera signal processing circuit 16. A detectedvalue indicative of brightness is, e.g., the integral of the pixelbrightness signal levels within the screen.

The computation unit 18 is formed from, e.g., a microcomputer, andcontrols various parts of the present apparatus on the basis of thebrightness and color distributions detected by the detector circuit 17and the video signals processed by the camera signal processing circuit16. The computation unit 18 computes and outputs, e.g., an exposuretiming control signal for controlling the exposure operation and thereading operation by each pixel of the image pickup device 13, a gaincontrol signal for controlling the gain of the AGC 14, a lens controlsignal for controlling the focusing of the lens 11 and the aperture ofthe iris 12 via the lens driver 22, and a flash control signal forcontrolling the firing operation of the flash 21 via the flashingcircuit 20. The memory unit 19 stores control data computed by thecomputation unit 18.

The flashing circuit 20 drives the flash 21 in accordance with the flashcontrol signal computed by the computation unit 18, for flash imaging.The flash 21 is lit in accordance with a driving signal from theflashing circuit 20. The lens driver 22 drives the lens 11 and the iris12 in accordance with the lens control signal computed by thecomputation unit 18. The memory unit 23 temporarily stores the videosignals (e.g., moving image information) processed by the camera signalprocessing circuit 16.

FIG. 2 is a diagram of a general configuration of a CCD image sensor,and FIG. 3 is a diagram of a general configuration of a CMOS imagesensor.

As shown in FIG. 2, the CCD image sensor includes a plurality of pixels31 arranged in a two-dimensional matrix-like configuration, as many Vtransfer registers 32 as the number of columns of the plurality ofpixels 31, and an H transfer register 33. Each pixel 31photoelectrically converts incident light into an analog video signal(charge). The plurality of V transfer registers 32 transfer thephotoelectrically converted pixel video signals, respectively, i.e.,vertically for each pixel (each line). The V transfer register 32transfers the video signals for as many pixels 31 as one linetransferred from the plurality of V transfer registers 32, horizontallyfor each pixel.

When the CCD image sensor is exposed to light, the light rays havingentered the pixels are photoelectrically converted into charges (videosignals), respectively. Each pixel 31 starts storing charge in an amountproportional to its incident light. When the CCD image sensor is given asignal for transferring the charge, the charges respectively stored inall the pixels 31 are transferred simultaneously to their respective Vtransfer registers 32. The charges in the pixels 31 (lines) transferredto the respective V transfer register 32 are transferred vertically foreach pixel (each line), to the H transfer register 33. The video signalsfor as many pixels as one line transferred to the H transfer register 33are then transferred horizontally and outputted therefrom, for eachpixel. The plurality of V transfer registers 32 and the H transferregister 33 are light-shielded. As a result, the pixel 31 charges oncetransferred thereto are kept constant, unexposed from light fromoutside.

As shown in FIG. 3, the CMOS image sensor includes a plurality of pixels41 arranged in a two-dimensional matrix-like configuration whereinpixels 41 in each column are electrically connected together, and acolumn 42 that transfers charges (video signals) transferred from any ofthe pixels 41 belonging to each column, horizontally for each pixel. TheCMOS image sensor does not have a component corresponding to the Vtransfer registers 32 in the CCD image sensor. Thus, each pixel 41 ofthe CMOS image sensor may have a larger surface area than each pixel 31of the CCD image sensor. Therefore, its dynamic range can be increasedto improve its sensitivity. The CMOS image sensor can be configured toselectively read pixels 41 at desired addresses. On the other hand, itcannot read pixels 41 belonging to the same column simultaneously, whichcomplicates their exposure and reading operations.

FIG. 4 is a timing chart showing an operation of the CCD image sensor,and FIG. 5 is a timing chart showing an operation of the CMOS imagesensor.

As shown in FIG. 4, the exposure operation of the CCD image sensor isperformed simultaneously for all the pixels 31. In the CMOS imagesensor, due to the absence of the component corresponding to the Vtransfer registers 32 of the CCD image sensor, when a certain pixel 41is read, other pixels are affected by ambient light, resulting inimproper storage of charges. As shown in FIG. 5, in the CMOS imagesensor, the exposure operation and the reading operation must have acertain relationship with each other for each pixel 41. In this example,a technique is employed to stagger the exposure start timing one line,for each pixel (each line) 41 in order to give all the pixels an equalexposure time.

FIG. 6 is a diagram showing a control flow of flash imaging.

As shown in FIG. 6, when a still image pickup mode is selected, anintegral of brightness signal levels contained in image signalsgenerated by the image pickup device 13 is detected by the detectorcircuit 17, and whether ambient light is high or low is determined bythe computation circuit 18 on the basis of the integral of thebrightness signal levels (step S1). When it is determined that theambient light is high, the process goes to step S2 to perform regularimaging without firing the flash 21. On the other hand, if it isdetermined in step S1 that the ambient light is low, the process goes tostep S3 to perform flash imaging. Note that the process goes to step S3to perform flash imaging regardless of the result of step S1 when anoperating mode is available which forcibly performs flash imaging in anyambient light conditions.

For flash imaging, first, the aperture of the iris 12, the exposure timefor the image pickup device 13 (shutter time), and the gain of the AGC14 are set (step S3). The aperture of the iris 12 is preferably set suchthat light from a near-range object does not exceed the dynamic range ofthe image pickup device 13 during a preflash by the flash 21.Pre-flashing is a process intended to compute the amount ofmain-flashing light. And if light exceeding the dynamic range enters theimage pickup device 13, distorted (saturated) video signals areproduced, disabling accurate computation of the amount of light for mainflashing. Moreover, the exposure time for the image pickup device 13 ispreferably set as short as possible. A longer exposure time results ingreater ambient light effects, which would narrow the dynamic range fordetecting the amount of pre-flashing light, thus reducing computationaccuracy for the amount of main-flashing light. The gain of the AGC 14is preferably set to such a smaller value as to reduce noise effects inthe video signals.

Next, while keeping the aperture, exposure time and gain settings,before-preflash exposure and reading operations are performed by theimage pickup device 13 without firing the flash 21, and abefore-preflash detected value (a), i.e., an integral of brightnesssignal levels contained in video signals is detected by the detectorcircuit 17, and stored in the memory unit 23. The before-preflashdetected value (a) means a detected value of only ambient light withoutpre-flashed light (step S4).

Next, while still keeping the aperture, exposure time and gain settings,a preflash is fired by the flash 21 in a predetermined amount of light(step S5). Then, during-preflash exposure and reading operations areperformed by the image pickup device 13, and a during-preflash detectedvalue (b), i.e., an integral of brightness signal levels contained invideo signals is detected by the detector circuit 17, and stored in thememory unit 23. The during-preflash detected value (b) means a detectedvalue of pre-flashed light and ambient light (step S6).

Next, the during-preflash detected value (b) and the before-preflashdetected value (a) stored in the memory unit 23 are read therefrom bythe computation circuit 18 to compute a differential detected valueobtained by subtracting the before-preflash detected value (a) from theduring-preflash detected value (b). The differential detected valuemeans a detected value containing only pre-flashed light with ambientlight excluded (step S7). Next, an amount of light for main flashing bythe flash 21 is computed by the computation circuit 18 on the basis ofthe differential detected value (step S8). Then, the flash 21 is firedin accordance with the amount of light computed, to perform flashimaging (step S9).

The before-preflash exposure operation in step S4 and theduring-preflash exposure operation in step S6 by the image pickup device13 are performed preferably as quickly as possible. Flash imaging isperformed generally under low illumination, but almost never withoutambient light. Moreover, for example, flash imaging is performed tohighlight a person who is seen dark due to backlight. Such ambient lightaffects the obtaining of correct detected values, particularly,during-preflash detected values (b), and this may result in obtainingincorrect differential detected values.

FIG. 7 is a diagram showing an example image of an object. Moreover,FIG. 8 is a diagram showing an example computation of a differentialdetected value under no ambient light, and FIG. 9 is a diagram showingan example computation of a differential detected value under ambientlight.

A description will be given of a result of a computation made as to anoutput from the image pickup device, i.e., a differential detected valuebased on detected values. The output is taken at a position defined bythe vertical dotted line shown in FIG. 7, when an image including around image A in the middle of the screen shown in FIG. 7 is formed byflash imaging.

As shown in FIG. 8, under no ambient light, the output from the imagepickup device before firing a preflash is zero, and thus itsbefore-preflash detected value (a) is also zero. At the time of thepreflash, a during-preflash detected value (b) exhibiting a large outputlevel for a portion corresponding to the image A is obtained. As aresult, a differential detected value coincides with the during-preflashdetected value (b).

As shown in FIG. 9, under ambient light, a before-preflash detectedvalue (a) corresponding to the ambient light is obtained. At the time ofthe preflash, the ambient light and pre-flashed light enter the imagepickup device. At this moment, any brightness signal exceeding the 100%output level of the image pickup device, i.e., any brightness signalexceeding the dynamic range of the image pickup device is clipped. As aresult, a differential detected value becomes smaller and distorted dueto the clipped portion being excluded therefrom. Therefore, a correctamount of light for main flashing cannot be computed. As mentionedearlier, flash imaging under almost no ambient light is rarelyperformed. Consequently, the exposure operations before and during apreflash must be performed as quickly as possible to reduce the ambientlight effects and thus to give a sufficient margin to the dynamic rangeof the image pickup device at the time of the preflash.

FIG. 10 is a diagram showing a flash imaging sequence for a still imageby a conventional CCD sensor, and FIG. 11 is a diagram showing a flashimaging sequence for a still image by a conventional CMOS sensor.Moreover, FIG. 12 is a diagram showing an image formed at the time of apreflash by the conventional CCD sensor, and FIG. 13 is a diagramshowing an image formed at the time of a preflash by the conventionalCMOS sensor.

As shown in FIG. 10, when the conventional CCD sensor is used as theimage pickup device, a before-preflash exposure is started at a time T1and a during-preflash exposure is started at a time T2, in a movingimage pickup mode. Detected values are obtained therefor, respectively,and their differential detected value is then computed. Subsequently,proceeding to the still image pickup mode, flash imaging by means ofmain flashing is performed. As shown in FIG. 12, pre-flashing in theduring-preflash exposure affects the entire area of the screen evenly.

As shown in FIG. 11, even when the conventional CMOS sensor is used asthe image pickup device, similarly, a before-preflash exposure isstarted at the time T1 and a during-preflash exposure is started at thetime T2, in the moving image pickup mode. Detected values are obtainedtherefor, respectively, and their differential detected value is thencomputed. However, as shown in FIG. 13, the preflash duration is soshort as 10 μsec that pre-flashing in the preflash exposure affects onlya limited area (upper zone) of the screen. As a result, from theremaining area (middle and lower zones) of the screen, one obtains adetected value representing only ambient light, without preflasheffects. Therefore, the during-preflash detected value cannot beobtained highly accurately, and hence the amount of light for mainflashing cannot be computed highly accurately, either.

FIG. 14 is a diagram showing a flash imaging sequence for a still imageby the CMOS sensor according to Embodiment 1.

As shown in FIG. 14, for starting a before-preflash exposure and aduring-preflash exposure, the imaging apparatus according to Embodiment1 of the present invention sweeps away charges from all the pixels ofits image pickup device 13, whereby the exposure operations are startedsimultaneously for all the pixels within the screen. Pre-flashing in theduring-preflash exposure operation affects the entire screen evenly fromtop to bottom. Therefore, a during-preflash detected value can beobtained highly accurately. On the other hand, the imaging apparatus ofthe present invention cannot read all the pixels within the screensimultaneously, and thus, the pixels are sequentially read one line at astaggered timing. Before-preflash and during-preflash exposure times aredesigned to last longer for pixels in lower lines, thus exposing pixelsin a first line for the shortest time (e.g., 1/4000 sec), and exposingpixels in a last line for the longest time (e.g., one over some hundredsof seconds).

The before-preflash and during-preflash exposure times differ from upperto lower zones of the screen. However, by setting shorter exposuretimes, the ambient light effects upon the during-preflash detected valuecan be reduced. Since pre-flashing is an operation originally intendedto compute the amount of main-flashing light, the pre-flashing isacceptable as long as it allows for accurate detection of the amount oflight reflected from an object during the preflash. Furthermore, theimaging apparatus according to Embodiment 1 detects a before-preflashdetected value in addition to a during-preflash detected value, andsubtracts the before-preflash detected value from the during-preflashdetected value, to obtain their differential detected value containingonly pre-flashed light with ambient light excluded. As a result, thedifference between the exposure times applied to the upper and lowerzones of the screen in the before-preflash and during-preflash exposuresis cancelled, whereby a differential detected value can be obtainedwhich contains only the pre-flashed light with ambient light excluded.Therefore, further more accurate differential detected values can beobtained, and further more accurate amounts of main-flashing light canbe obtained.

FIG. 15 is a diagram showing an example computation of a differentialdetected value in Embodiment 1.

A description will be given of a result of a computation made as to anoutput from the image pickup device, i.e., a differential detected valuebased on detected values, similarly to the differential detected valuesshown in FIGS. 8 and 9. The output is taken at the position defined bythe vertical dotted line shown in FIG. 7, when the image including theround image A in the middle of the screen shown in FIG. 7 is formed byflash imaging.

As shown in FIG. 15, before a preflash, pixels in the upper zone of thescreen are exposed for so short a period as 1/4000 sec, and thus, theimage output level is almost zero. And pixels positioned in lower linesof the screen have longer exposure times, and thus ambient light effectsgradually grow large in the image output level. During the preflash, aportion corresponding to the image A produces a large output level, andis also affected by the ambient light. However, since the amount ofambient light contained in the incident light both before and during thepreflash is smaller than the amount of pre-flashing light, and thus, onecan obtain such an image output level as not to exceed the 100% outputlevel of the image pickup device, i.e., the dynamic range of the imagepickup device.

As described above, according to the imaging apparatus of Embodiment 1,during a preflash operation, an exposure operation is startedsimultaneously for all the pixels of the image pickup device 13 to forman image at the time of the preflash. Thus, the preflash effects can beextended over the entire area of the image pickup device, while reducingthe exposure times for the image pickup device. Therefore, this isadvantageous in obtaining the amount of main-flashing light with highaccuracy. Moreover, the preflash time of the flash 21 is set to such asmall value that the amount of light entering the image pickup device 13does not exceed the dynamic range of the image pickup device 13. As aresult, an undistorted image faithfully reproducing the amount ofincident light can be formed by the image pickup device 13. Thus, thisis also advantageous in obtaining the amount of main-flashing light witheven higher accuracy.

Furthermore, by detecting a before-preflash detected value in additionto a during-preflash detected value, the before-preflash detected valueis subtracted from the during-preflash detected value, to obtain theirdifferential detected value containing only pre-flashed light withambient light excluded. As a result, the differential detected valuecontaining only the pre-flashed light with the ambient light excludedcan be obtained. Consequently, this is further advantageous in obtainingthe amount of main-flashing light with high accuracy.

Note that in FIG. 14, during a period in which before-preflash andduring-preflash operations are performed by the image pickup device 13,output from the image pickup device 13 occurs at intervals of 2 Vs(every two vertical sync signals), providing only incomplete videosignals (moving image). Additionally, affected by the difference inexposure time from one zone to another within the screen, a distortedvideo is outputted from the image pickup device during this period. Inorder to overcome this shortcoming, the following configuration may beimplemented. That is, video signals generated at a timing some Vs aheadof this period are stored in the memory unit 23 beforehand, and whenstarting a before-preflash exposure operation, the image pickup device13 reads the video signals generated at the timing some Vs ahead whichhave been stored in the memory unit 23, instead of an image to be formedby the image pickup device 13, and outputs the read video signals to adownstream image recording system or image output system. This permits auser to be unaware of distortions in the video.

INDUSTRIAL APPLICABILITY

According to the imaging apparatus of the present invention, during apreflash operation by a flash, an exposure operation is startedsimultaneously for all the pixels of the image pickup device, wherebypreflash effects can be extended over the entire area of the imagepickup device, while reducing the exposure times such that the amount oflight entering the entire area of the image pickup device does notexceed the dynamic range of the image pickup device as much as possible.Consequently, this is advantageous in obtaining the amount ofmain-flashing light with high accuracy.

1. An imaging apparatus comprising a flash for emitting light onto anobject, an image pickup device formed from a plurality of pixels, eachof which can perform any of an exposure operation and a readingoperation thereof at a timing different from that of the other, adetector circuit for detecting a brightness of image information formedby the image pickup device, and a control circuit for controllingoperations of the image pickup device and of the detector circuit, saidimaging apparatus characterized in that: said control circuit causes theflash to fire a preflash before a main-flashing operation by the flash,causes the image pickup device to form an image at the time of thepreflash, and causes the detector circuit to detect the brightness ofimage information formed at the time of the preflash, to compute anamount of main-flashing light to be fired by said flash on the basis ofthe detected brightness of the image information formed at the time ofthe preflash; and said control circuit causes starting the exposureoperation simultaneously for all the pixels of said image pickup deviceat the time of the preflash by said flash, whereby to form the image atthe time of the preflash.
 2. The imaging apparatus as described in claim1, characterized in that: said apparatus further comprises a memory unitfor storing image information formed by said image pickup device beforea pre-flashing operation by the flash; and when said pickup devicestarts a during-preflash exposure operation, said control circuit causessaid pickup device to read the image information which have been storedin said memory unit, instead of an image to be formed by said imagepickup device at the time of the preflash, and outputs the read imageinformation to a downstream image recording apparatus or image outputapparatus.
 3. An imaging apparatus comprising a flash for emitting lightonto an object, an image pickup device formed from a plurality ofpixels, each of which can perform any of an exposure operation and areading operation thereof at a timing different from that of the other,a detector circuit for detecting a brightness of image informationformed by the image pickup device, and a control circuit for controllingoperations of the image pickup device and of the detector circuit, saidimaging apparatus characterized in that: said control circuit causes theflash to fire a preflash before a main-flashing operation by the flash,causes the image pickup device to form an image at the time of thepreflash, causes the detector circuit to detect the brightness of imageinformation formed at the time of the preflash, causes the image pickupdevice to form a before-preflash image with said flash not fired beforethe preflash operation by said flash, and causes the detector circuit todetect the brightness of image information formed before the preflash,to compute a differential value obtained from the brightness of imageinformation formed before the preflash and the brightness of imageinformation formed during the preflash respectively detected by saiddetector circuit, and compute an amount of main-flashing light to befired by the flash on the basis of the computed differential value; andsaid control circuit causes starting the exposure operationsimultaneously for all the pixels of said image pickup device before apreflash operation and during a preflash operation by said flash,whereby to form the images before the preflash operation and during thepreflash operation.
 4. The imaging apparatus as described in claim 3,characterized in that: said apparatus further comprises a memory unitfor storing image information formed by said image pickup device beforesaid image pickup device forms an image before the preflash; and whensaid pickup device starts a before-preflash exposure operation, saidcontrol circuit causes said pickup device to read the image informationwhich have been stored in said memory unit, instead of images to beformed by said image pickup device before the preflash and during thepreflash, and outputs the read image information to a downstream imagerecording apparatus or image output apparatus.
 5. The imaging apparatusas described in claim 3, characterized in that said image pickup devicecomprises an XY addressable image sensor.
 6. The imaging apparatus asdescribed in claim 5, characterized in that said XY addressable imagesensor comprises a CMOS image sensor.